docs: def build_null_stop_time_series(
      feed: "Feed",
      date_label: str = "20010101",
      freq: str = "5Min",
      *,
      split_directions: bool = False,
  ) -> DataFrame:
      """
      Return a stop time series with the same index and hierarchical columns
      as output by :func:`compute_stop_time_series_base`,
      but fill it full of null values.
      """
      start = date_label
      end = pd.to_datetime(date_label + " 23:59:00")
      rng = pd.date_range(start, end, freq=freq)
      inds = ["num_trips"]
      sids = feed.stops.stop_id
      if split_directions:
          product = [inds, sids, [0, 1]]
          names = ["indicator", "stop_id", "direction_id"]
      else:
          product = [inds, sids]
          names = ["indicator", "stop_id"]
      cols = pd.MultiIndex.from_product(product, names=names)
      return pd.DataFrame([], index=rng, columns=cols).sort_index(
          axis=1, sort_remaining=True
      )